According to the Kumano faith, this large boulder, wedged on the side of Gongen-yama, was where the three principal gods descended to earth. It was the original site of worship, before Kumano Hayatama Taisha was built nearby. It's a somewhat tricky climb up the 538 uneven steps to the rock, where there's a small shrine, Kamikura-jinja.
It's about a 15-minute walk south to the start of the climb from Kumano Hayatama Taisha.